This invention relates to matrix printing heads for use in type printers in data terminals, typewriters, bookkeeping and other office and similar machines using the "thermoelectric" printing method wherein electrically conductive printing ink is transferred to a record carrier by applying an electrical potential gradient to preselected positions on a carrier of the printing ink.
U.S. patent application Ser. No. 493,205 of Rainer Anton and Reinholt Gebhardt filed July 31, 1974, discloses a method of and apparatus for impactless or non-mechanical printing. This method and apparatus uses an electrically conductive ink, a screen-like carrier for the ink, a record carrier and two electrodes to impress an electric potential gradient across the ink. One of these electrodes, known as the printing head, is capable of imposing an electrical potential gradient having a preselected configuration, generally an alphanumeric character. When a preselected character is to be printed, the printing head impresses an electrical potential gradient having the configuration of the character between the head and the screen electrode, to form a current path across the ink carrier and through the electrically conductive ink. This current path may either generate a joulean heat through the conductive ink, which will soften or locally liquefy the ink for transfer to the record carrier, or else may cause discharges between certain particles within the conductive ink to charge these particles and at the same time heat and soften surrounding particles so that the charged particles may be moved and/or accelerated by the electric field to transfer the ink to the record carrier. In either event, when an electric field of appropriate strength is impressed across the ink between the printing head and the screen electrode, a defined particle transport toward the screen electrode takes place. A record carrier, positioned at the outer surface of the screen electrode, absorbs the ink which passes through the screen electrode, thereby recording the preselected character.
It will be noted that the printing electrodes of the above-described method and apparatus do not provide electrostatic charges as do other electrical printing processes, but rather serve primarily to generate a current path or current flow through the conductive ink.
German disclosure No. 2,137,371 of Feb. 3, 1972, in the ICP Patent class G 06 K, 15/14, a "Method for the Manufacture of a Printing Head with Wire Printing Electrodes," describes a method for manufacturing electrostatic printing heads in which wire printing pins or electrodes are embedded in an adhesive to form a wire and adhesive foil, which is then disposed between two opposing end plates. In addition, a number of wire and adhesive foils may be disposed between the end plates to form a multilayered printing head. According to this method, closely spaced turns of wire are wound on a revolving drum and become embedded in an adhesive carrier which has been applied to the drum. The cylinder which is thus formed is split open, removed from the drum and spread out to form a flat foil, whose wire side is then brought in contact with and bonded to an epoxy resin end plate.
Printing heads made by this method are claimed to have many advantages over previous printing heads for use in electrostatic printers of recording, registering and measuring instruments. These heads, it is claimed, are more durable, cheaper to produce, increase the printing electrode density, produce heads of greater strength than heretofore produced, and provide better control over the electrode spacing and changes in dimension of the electrodes.
It has been found, however, that printing heads manufactured by the above-described method offer no advantages when used as impactless thermoelectric printing heads according to the method and apparatus described in the aforementioned U.S. application Ser. No. 493,205. In printing according to the impactless thermoelectric method, due to the basic difference in the phenomenon employed, the density of the electrodes in the head or matrix is much less significant than the accurate adjustment of the electrodes within the matrix and their matrix position relative to the screen electrode and ink carrier. To insure proper transfer of the ink to the record carrier it is desirable that the grid pattern of the screen electrode be dimensioned such that a single conductor of the printing head is located symmetrically or centrally behind one or more associated mesh openings. While the dimensions of the grid pattern of the screen electrode may match those of the printing head conductor matrix, this is not necessary to comply with the requirement stated above.
Thus, the wound wire method of making heads as described in German disclosure No. 2,137,371 is inappropriate to produce thermoelectric matrix printing heads, since the grid pattern of the electrodes produced by winding wire on a drum cannot be sufficiently accurately controlled, and the use of wire for electrodes, and wire and adhesive foil for a matrix similarly cannot produce a matrix of sufficient dimensional accuracy. On the other hand, manufacturing steps and techniques which would be inappropriate in making the wound wire head may be used in making the thermoelectric matrix printing head. Thus, to effectively employ the "thermoelectric" impactless printing process, a need exists for a printing head and for a method of making a printing head which is dimensionally accurate and repeatable and which is inexpensive and efficient to manufacture.